Retainer ring for semiconductor manufacturing processes

ABSTRACT

An embodiment retainer ring includes an outer ring encircling an opening and an inner ring attached to the outer ring. The inner ring is disposed between the opening and the outer ring. The inner ring includes a softer material than the outer ring and a plurality of voids within the softer material.

BACKGROUND

Generally, semiconductor devices comprise active components, such as transistors, formed on a substrate. Any number of interconnect layers may be formed over the substrate connecting the active components to each other and to outside devices. The interconnect layers are typically made of low-k dielectric materials comprising metallic trenches/vias.

As the layers of a device are formed, it is sometimes desirable to planarize the device. For example, the formation of metallic features in the substrate or in a metal layer may cause uneven topography. This uneven topography creates difficulties in the formation of subsequent layers. For example, uneven topography may interfere with the photolithographic process used to form various features in a device. It is, therefore, desirable to planarize the surface of the device after various features or layers are formed. One method of planarization is chemical mechanical polishing (CMP).

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A, 1B, and 1C illustrate varying views of a retainer ring in accordance with some embodiments.

FIG. 2 illustrates a cross sectional view of the retainer ring during a semiconductor process in accordance with some embodiments.

FIGS. 3A and 3B illustrate a micrographic view of semiconductor wafer edges after processing.

FIG. 4 illustrates a process flow of a semiconductor process using a retainer ring according to some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Various embodiments are described with respect to a specific context, namely a retainer ring for use during a chemical mechanical polish (CMP) process. However, those skilled in the art will recognize that the described retainer ring could provide advantageous features in other manufacturing processes, particularly processes where it is desirable to provide a relatively soft interface to protect wafer edges during processing and/or handling steps.

Various embodiments include a retainer ring for securing a wafer during processing (e.g., a CMP process). The retainer ring may include an outer ring and an inner ring attached to a sidewall of the outer ring. Compared to the material of the outer ring, the inner ring may comprise a relatively soft material. In some embodiments, the relatively soft material of the inner ring may be porous. During processing, the inner ring is disposed between a wafer and the relatively hard material of the outer ring in order to protect the edges of the wafer from colliding with the outer ring. Thus, wafer bevel peeling and scratches resulting from collisions between the wafer and the outer ring can be advantageously reduced. Furthermore, in embodiments where the inner ring comprises a porous material, the voids within the inner ring may absorb collision energy and protect the wafer during processing, which may further reduce manufacturing defects.

FIG. 1A is a schematic diagram of an embodiment retainer ring 100 for CMP according to some embodiments. The retainer ring 100 includes an outer ring 102 and an inner ring 104 attached to the outer ring 102. In some embodiments, the inner ring 104 is attached to the inside of the outer ring 102 using an adhesive (e.g., glue) layer 110 at the interface between the outer ring 102 and the inner ring 104. In other embodiments, the inner ring 104 may be secured to the outer ring 102 using a different mechanism.

The retainer ring 100 is generally annular in shape with an opening 108 disposed in a center portion. The outer ring 102 and the inner ring 104 encircle the opening 108, with the inner ring 104 being disposed between the outer ring 102 and the opening 108. During processing, a wafer may be disposed within opening 108 of the retainer ring 100, and the retainer ring 100 may secure the wafer as explained in greater detail below. FIG. 1B is a sliced cross-sectional view of the embodiment retainer ring 100 in FIG. 1A taken along the line 106 according to some embodiments. FIG. 1C illustrates a detailed cross-sectional view of the inner ring 104 according to some embodiments.

In various embodiments, the inner ring 104 comprises a softer material than the outer ring 102. In some embodiments, the inner ring 104 has a hardness less than about 55 in Shore A hardness scale or less than about 20 in Shore B hardness scale. Shore hardness is a measure of the resistance of a material to penetration of a spring loaded needle-like indenter (sometimes known as a durometer). Hardness of polymers (rubbers, plastics) may be measured by Shore scales. Shore A scale is used for testing soft elastomers (e.g., rubbers) and other soft polymers. Hardness of hard elastomers and most other polymer materials may be measured by Shore D scale. Different indenter shapes and different spring loads may be used for different Shore scales (e.g., A and D). For example, the loading force of Shore A may be about 822 g, and the loading force of Shore D may be about 4536 g. Shore hardness values may vary in range from 0 to 100 with a maximum hardness value of 100 corresponding to zero penetration. It has been observed that by using a relatively soft material for the inner ring 104 within the above range, wafer edge damage (e.g., scratches and/or peeling) during processing can be reduced.

The material of the inner ring 104 may be selected so that the inner ring 104 is relatively impervious to damage during processing. For example, during CMP, a chemical slurry may be applied to a wafer held by the retainer ring 100 (see e.g., FIG. 2), and the retainer ring 100 may also be exposed to the slurry. The material of the inner ring 104 may be selected to avoid damage by the slurry environment (e.g., a low pH environment, a high pH environment, an oxidant containing environment, and the like). In an embodiment, the inner ring 104 comprises a polymer material, such as, polyurethane although other suitable materials (e.g., having a Shore hardness value as described above) may be used as well. In contrast, the outer ring 102 may comprise a relatively hard material (e.g., harder than the inner ring 104). For example, in an embodiment, the outer ring 102 may comprises polyether ether ketone (PEEK), polyphenylene sulfide (PPS), any combination thereof, or any other suitable material. In some embodiments, outer ring 102 has a hardness greater than about 85 in Shore D hardness scale. It has been observed that when the hardness of outer ring 102 falls within the above range, outer ring 102 provides improved chemical and wear resistance and advantageously increases the durability of retainer ring 100.

The inner ring 104 may further comprise a porous material, which may include pores (e.g., voids 104′) as illustrated by FIG. 1C. Voids 104′ may be formed, for example, by solvent casting combined with particle leaching (e.g., leaching solid particles, such as porogens, from a polymer solution), thermally-induced phase separation, melt molding (e.g., molding with polymer powder and porogen at a high temperature, such as higher than the polymer glass-transition temperature and leaching out the porogen), gas foaming, emulsion freeze drying (e.g., cooling a polymer solution to form solvent ice crystals and removing the solvent by using a pressure lower than an equilibrium vapor pressure of the solvent), or the like. In some embodiments, a specific gravity of the inner ring 104 may be less than about 1.0, such as less than about 0.8, for example. Voids 104′ may be used to absorb energy (e.g., when a wafer collides with the retainer ring 100) during processing. Thus, the inner ring 104 may protect edges of the wafer from damage caused by collisions during processing. For example, it has been observed that when the specific gravity of the inner ring 104 is within the above range, damage to a wafer during processing may be advantageously reduced.

In some embodiments, the inside diameter D1 of the retainer ring 100 ranges 300 mm to 303 mm, the outside diameter D2 ranges from 329 mm to 333 mm. The size of the retainer ring 100 may be selected based on a size of a wafer the retainer ring 100 secures during processing. For example, in other embodiments, the size of the retainer ring 100 can be different, e.g. being sized to accommodate a 450 mm diameter wafer during a CMP process or other process where the wafer is retained during a process step.

In some embodiments, the inner ring 104 has a thickness T1 ranging from about 0.1 mm to about 0.7 mm. Furthermore, the inner ring 104 may have a height T2 of about 0.8 mm to about 20 mm. The height of the inner ring 104 may be selected based on a height of a wafer secured by the retainer ring 100 during processing. For example, in other embodiments, the height of the inner ring 104 may be different, e.g., being sized to accommodate a taller wafer during a CMP process or other process where the wafer is retained during a process step. Thus, the inner ring 104 may be disposed between all sidewalls of the wafer and the outer ring 102 during processing in order to protect the wafer and reduce damage to wafer edges.

In some embodiments, height T2 of the inner ring 104 may be less than a height of the outer ring 102 in an embodiment. For example, in FIG. 1B, a top surface of the inner ring 104 is lower than a top surface of the outer ring 102. In other embodiments, heights of the inner ring 104 and the outer ring 102 may be substantially equal.

FIG. 2 illustrates the retainer ring 100 of Fig. lA in use during a CMP process according to some embodiments. In FIG. 2, a retainer ring 100 including the outer ring 102 and the retainer ring 100 is mounted to a carrier head 202 using mechanical fasteners such as screws or by any other suitable means. As described above, the inner ring 104 is softer than the outer ring 102. In some embodiments, the inner ring 104 has a hardness ranging from 15 to 105 in Shore A hardness scale. Furthermore, the inner ring 104 may comprise a porous material to further protect edges of the wafer 200 during processing. For example, the porous material may act as a shock absorber. In some embodiments, the inner ring 104 comprises porous polyurethane, or any other suitable material, the outer ring 102 comprises polyether ether ketone (PEEK), polyphenylene sulfide (PPS), any combination thereof, or any other suitable material.

The carrier head 202 has a membrane 204 that interfaces with a wafer 200. The wafer 200 may be a semiconductor wafer comprising, for example, a semiconductor substrate (e.g., comprising silicon although other III-V semiconductor materials may be used as well), active devices (e.g., transistors) disposed at a top surface of the semiconductor structure, and/or various interconnect structures. The interconnect structure may include conductive features, which electrically connect the active devices in order to form functional circuits. In various embodiments, CMP processing may be applied to the wafer 200 during any stage of manufacture in order to planarize, reduce, or remove features (e.g., dielectric material, semiconductor material, and/or conductive material) of the wafer 200. Thus, the wafer 200 being processed may include any subset of the above features as well as other features.

Initially, the carrier head 202 may be lowered towards a wafer 200 placed on a stage (not illustrated). The carrier head 202 may pick up the wafer 200 from the stage using vacuum suction on the membrane 204 so that the wafer 200 is disposed within an opening 108 of the retainer ring 100. While the wafer 200 is held by the carrier head 202, the inner ring 104 may be disposed between edges of the wafer 200 and the outer ring 102. Thus, the inner ring 104 acts as a buffer between the wafer 200 and the outer ring 102 in order to reduce edge damage (e.g., peeling, scratches, and the like) caused by collisions between the wafer 200 and the retainer ring 100, which may occur during processing.

The carrier head 202 may carry the wafer 200 to a polishing pad 206 disposed over a platen 208. In an embodiment the polishing pad 206 may be a single layer or composite layer of materials such as polyurethane or polyurethane mixed with fillers, and may have a hardness of about 50 or less on the Shore D Hardness scale. The surface of the polishing pad 206 may be a roughened surface with micropores within it. However, any other suitable polishing pad may alternatively be used to planarize the wafer 200.

The carrier head 202 may be lowered towards the polishing pad 206 for polishing the wafer 200. The wafer 200 is positioned so that the surface to be planarized faces downward towards the polishing pad 206. Other methods of disposing the wafer 200 over the polishing pad 206 may be used as well. For example, in another embodiment, the wafer 200 may be placed on the polishing pad 206 using a different mechanism, and the carrier head 202 may be lowered onto the wafer 200 while the wafer 200 is on the polishing pad 206.

During CMP, the membrane 204 inside the carrier head 202 is pressurized to push the wafer 200 onto the polishing pad 206. The wafer 200 is polished by rotating the carrier head 202 and/or the polishing pad 206/platen 208 as indicated by arrows 210. Although FIG. 2 illustrates a particular direction of rotation, the carrier head 202 and/or the platen 208 may be rotated in either directing during processing, and the rotation direction of the carrier head 202 and/or the platen 208 may or may not be the same. By rotating the wafer 200 against the polishing pad 206, the polishing pad 206 mechanically grinds a surface of the wafer 200 to remove undesirable wafer material and planarize the wafer 200.

During CMP, a chemical slurry is dispensed over a top surface of the polishing pad 206 by a slurry dispenser (not illustrated). Although not explicitly illustrated, in an embodiment, a gap may be disposed between the retainer ring 100 and the polishing pad 206 during CMP to allow the slurry to be distributed under a bottom surface (e.g., a surface to be planarized) of the wafer 200. In other embodiments, the retainer ring 100 may contact the polishing pad 206, and the retainer ring 100 may include one or more grooves extending from an outer sidewall to an inner sidewall in order to dispense slurry onto the wafer 200. As discussed above, a material of the retainer ring 100 (e.g., the outer ring 102 and the inner ring 104) may be selected to be undamaged by the slurry environment. For example, the inner ring 104 may comprise porous polyurethane.

The composition of the slurry depends on the type of material on the wafer surface undergoing CMP. For example, the slurry may comprise a first reactant, an abrasive, a first surfactant, and a solvent. The first reactant may be a chemical that will chemically react with a material of the wafer 200 (e.g., a conductive material) in order to assist the polishing pad 206 in grinding away the material, such as an oxidizer. In an embodiment in which the material is tungsten, the first reactant may be hydrogen peroxide, although any other suitable reactant, such as hydroxylamine, periodic acid, ammonium persulfate, other periodates, iodates, peroxomono, sulfates, peroxymonosulfuric acid, perborates, malonamide, combinations of these, and the like, that will aid in the removal of the material may alternatively be utilized. Other reactants may be used in order to remove other materials. As another example, in an embodiment in which the material is an oxide, the first reactant may comprise an HNO₃ reactant.

The abrasive may be any suitable particulate that, in conjunction with the polishing pad 206, aids in the planarization of the wafer 200. In an embodiment the abrasive may be silica (e.g., silicon oxide). However, any other suitable abrasive, such as aluminum oxide, cerium oxide, polycrystalline diamond, polymer particles such as polymethacrylate or polymethacryclic, combinations of these, or the like, may alternatively be utilized and are fully intended to be included within the scope of the embodiments.

The first surfactant may be utilized to help disperse the first reactant and abrasive within the CMP slurry and also prevent (or at least reduce) the abrasive from agglomerating during the CMP process. In an embodiment the first surfactant may include sodium salts of polyacrylic acid, potassium oleate, sulfosuccinates, sulfosuccinate derivatives, sulfonated amines, sulfonated amides, sulfates of alcohols, alkylanyl sulfonates, carboxylated alcohols, alkylamino propionic acids, alkyliminodipropionic acids, potassium oleate, sulfosuccinates, sulfosuccinate derivatives, sulfates of alcohols, alkylanyl sulfonates, carboxylated alcohols, sulfonated amines, sulfonated amides, alkylamino propionic acids, alkyliminodipropionic acids, combinations of these, or the like. However, these embodiments are not intended to be limited to these surfactants, as any suitable surfactant may alternatively be utilized as the first surfactant.

The remainder of the CMP slurry may be a solvent that may be utilized to combine the first reactant, the abrasive, and the first surfactant and allow the mixture to be moved and dispersed onto the polishing pad 206. In an embodiment the solvent of the CMP slurry may a solvent such as deionized water or an alcohol. However, any other suitable solvent may alternatively be utilized.

Furthermore, a pad conditioner arm (not illustrated) may move a rotating pad conditioning head (not illustrated) in a sweeping motion over a region of the polishing pad 206 during CMP. The conditioning head holds a pad conditioner in contact with the polishing pad 206. The pad conditioner may comprise a substrate over which an array of abrasive particles, such as diamonds, is bonded using, for example, electroplating. The pad conditioner may be used to remove built-up wafer debris and excess slurry from the polishing pad 206. The pad conditioner may also acts as an abrasive for the polishing pad 206 to create an appropriate texture against which the wafer 200 may be mechanically ground.

The CMP process may be a one-step CMP process (e.g., where a single polishing pad 206 is used) or a multi-step CMP process. For example, the polishing pad 206 may be used during a bulk CMP process. In such embodiments, the wafer 200 may be removed from the polishing pad 206 and may be transferred to a buffing polishing pad (not illustrated). The buffing polishing pad may perform a similar CMP process as described above, with the second polishing pad grinding away a surface of the wafer 200 and a buffing slurry being dispersed to aid in the grinding process. The buffing slurry may be selected based on a material of the wafer 200 being planarized.

In an embodiment the second polishing pad may be a soft buffing pad which may planarize the wafer 200 at a slower and more controlled rate than the first polishing pad 206 while also buffing and eliminating defects and scratches that may have been caused by the bulk CMP process. In an embodiment the second polishing pad may be rotated relative to the wafer 200 while the buffing slurry is dispensed on the second polishing pad. The buffing CMP process may be continued until desired materials have been removed from the surface of the wafer 200. In some embodiments, a timed or optical end-point detection may be used to determine when to stop the polishing on the wafer 200.

During the various CMP processes described above, the wafer 200 is confined within the inner ring 104 during the polishing. With the retainer ring 100, the softer inner ring 104 absorbs impact/contact energy and reduces vibrations between the retainer ring 100 and the wafer 200 during the CMP process and prevents damage/peeling on the wafer 200. For example, FIG. 3A illustrates a micrographic view of a semiconductor wafer edge 300 after processing using a retainer ring without a soft inner ring, and FIG. 3B illustrates a micrographic view of a semiconductor wafer edge 350 after processing using a retainer ring having a soft inner ring (e.g., inner ring 104 as described above). Comparing the edge 300 with the edge 350, fewer defects (e.g., scratches and/or peeling) are formed in the wafer edge 350 where a soft inner ring was used than in the wafer edge 300 where the soft inner ring was omitted.

FIG. 4 illustrates a process flow 400 of a semiconductor process in accordance with some embodiments. In step 402, a retainer ring is provided. Retainer ring includes an outer ring and an inner ring attached to the outer ring. In some embodiments, the inner ring comprises a relatively soft, porous material (e.g., porous polyurethane), and a material of the inner ring may be softer than a material of the outer ring. In contrast, the material of the outer ring may be harder than the material of the inner ring. For example, the outer ring may comprise polyether ether ketone (PEEK), polyphenylene sulfide (PPS), any combination thereof, or any other suitable material.

The inner ring encircles a hollow, center area of the retainer ring, and a wafer may be secured in the hollow center area of the retainer ring during processing. In step 406, a wafer is positioned within the retainer ring (e.g., within the hollow center area of the retainer ring). The inner ring may be disposed between edges of the wafer and the outer ring while the wafer is secured by the retainer ring. The inner ring may be at least as tall as the wafer in order to protect wafer edges during processing. In step 406, processing is performed on the wafer while the wafer is secured by the retainer ring. For example, a CMP process may be performed while the retainer ring secures the wafer. The retainer ring may be secured to a polish head during processing in order to position and rotate the wafer against a polishing pad as described above. Furthermore, a chemical slurry may be dispensed under the wafer (e.g., under the retainer ring and/or through grooves in the retainer ring) during processing. In some embodiments, the material of the retainer ring may be selected to be resilient to damage from a chemical slurry environment. In various embodiments, the inner ring may protect edges of the wafer from colliding with the relatively hard material of the outer ring during processing. In embodiments where the inner ring comprises a porous material, the voids in the material may facilitate shock absorption, which further reduces damage to the wafer edges. Thus, damage (e.g., bevel edge peeling and/or scratches) to the wafer edge may be advantageously reduced.

Various embodiments include a retainer ring for securing a wafer during processing (e.g., a CMP process). The retainer ring may include an outer ring and an inner ring attached to an inner sidewall of the outer ring. Both the outer ring and the inner ring may encircle an opening with the inner ring being disposed between the outer ring and the opening. The inner ring may comprise a relatively soft material, which may be porous. During processing, the inner ring is disposed between a wafer secured by the retainer ring and the relatively hard material of the outer ring. Thus, the inner ring may protect edges of the wafer from colliding with the outer ring. By including an inner ring, wafer bevel peeling and scratches resulting from collisions between the wafer and the outer ring can be advantageously reduced. Furthermore, in embodiments where the inner ring comprises a porous material, the voids within the inner ring may absorb collision energy and protect the wafer during processing, which may further reduce manufacturing defects.

In accordance with an embodiment, a retainer ring includes an outer ring encircling an opening and an inner ring attached to the outer ring. The inner ring is disposed between the opening and the outer ring. The inner ring includes a softer material than the outer ring and a plurality of voids within the softer material.

In accordance with another embodiment, a polishing apparatus includes a polishing head and a retainer ring attached to the polishing head. The retainer ring includes an outer ring including a first material and an inner ring including a porous second material having a specific gravity less than about 1.0. The porous second material is softer than the first material. The inner ring is disposed between edges of a wafer and the outer ring when the retainer ring secures a wafer.

In accordance with yet another embodiment, a method for processing a wafer includes providing a retainer ring having an outer ring encircling an opening and an inner ring attached to the outer ring and encircling the opening. The inner ring includes a different material than the outer ring, and the different material is porous. The method also includes positioning a wafer in the opening. The inner ring is disposed between edges of the wafer and the outer ring. The method also includes processing the wafer while the wafer is secured by the retainer ring.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A retainer ring comprising: an outer ring encircling an opening; and an inner ring attached to the outer ring, wherein the inner ring is disposed between the opening and the outer ring, and wherein the inner ring comprises: a softer material than the outer ring; and a plurality of voids within the softer material.
 2. The retainer ring of claim 1, wherein the inner ring is attached to the outer ring by an adhesive layer disposed between the inner ring and the outer ring.
 3. The retainer ring of claim 1, wherein the inner ring comprises polyurethane.
 4. The retainer ring of claim 1, wherein the outer ring comprises polyether ether ketone (PEEK), polyphenylene sulfide (PPS), or a combination thereof.
 5. The retainer ring of claim 1, wherein a specific gravity of the inner ring is less than about 1.0.
 6. The retainer ring of claim 1, wherein a hardness of the inner ring is less than about 55 in a Shore A hardness scale or about 20 in a Shore B hardness scale.
 7. The retainer ring of claim 1, wherein the inner ring comprises a top surface lower than a top surface of the outer ring.
 8. The retainer ring of claim 1, wherein the retainer ring is configured to secure a wafer in the opening, and wherein a height of the inner ring is at least a height of the wafer.
 9. A polishing apparatus comprising: a polishing head; and a retainer ring attached to the polishing head, wherein the retainer ring comprises: an outer ring comprising a first material; and an inner ring comprising a porous second material having a specific gravity less than about 1.0, wherein the porous second material is softer than the first material, and wherein the inner ring is disposed between edges of a wafer and the outer ring when the retainer ring secures a wafer.
 10. The polishing apparatus of claim 9, wherein the porous second material comprises polyurethane, and wherein the first material comprises polyether ether ketone (PEEK), polyphenylene sulfide (PPS), or a combination thereof.
 11. The polishing apparatus of claim 9, wherein the specific gravity of the porous second material is less than about 0.8.
 12. The polishing apparatus of claim 9, wherein the porous second material is less than about 55 in a Shore A hardness scale or less than about 20 in a Shore B hardness scale.
 13. The polishing apparatus of claim 9, wherein the polishing head further comprises a membrane configured to apply downward pressure on a wafer secured by the retainer ring.
 14. The polishing apparatus of claim 9 further comprising a polishing pad, wherein the polishing head is configured to rotate a wafer secured by the retainer ring against the polishing pad.
 15. The polishing apparatus of claim 9, wherein the retainer ring is attached to the polishing head using mechanical screws.
 16. A method for processing a wafer comprising: providing a retainer ring comprising: an outer ring encircling an opening; and an inner ring attached to the outer ring and encircling the opening, wherein the inner ring comprises a different material than the outer ring, and wherein the different material is porous; positioning a wafer in the opening, wherein the inner ring is disposed between edges of the wafer and the outer ring; and processing the wafer while the wafer is secured by the retainer ring.
 17. The method of claim 16, wherein the inner ring comprises a softer material than the outer ring.
 18. The method of claim 16, wherein the inner ring comprises polyurethane, and wherein the outer ring comprises polyether ether ketone (PEEK), polyphenylene sulfide (PPS), or a combination thereof.
 19. The method of claim 16, wherein a specific gravity of the inner ring is less than about 1.0.
 20. The method of claim 16, wherein processing the wafer comprises applying a chemical mechanical polish (CMP) process to the wafer. 